Vaccines to combat infectious diseases
Slides: https://www.andreashandel.com/presentations/
2025-10-29
Question
What is the main mechanism by which a vaccine protects from infection:
- It induces a protective metabolic state
- It induces a protective immune response
- It induces a protective disease avoidance state
- All of A) - C)
- None of A) - C)
About me
- Professor of Epidemiology and Biostatistics in the College of Public Health at the University of Georgia.
- Modeling and analysis of infectious diseases, mainly influenza and norovirus.
- Work on better understanding the spread and control of infectious diseases.
- Teach courses on infectious diseases, data analysis/biostatistics and epidemiology.
More about me
- Originally from Germany, I moved to the US for graduate school. I have now lived in the US longer than in Germany.
- I trained as a physicist, I have never taken an epidemiology or biostatistics class in my life.
- If I don’t work, I grow, cook and eat, and mountain bike.
Vaccines are pretty good
![]()
xkcd.com
Vaccines are pretty good
![]()
Pollard et al 2021
The idea behind vaccines
Infection
Healthy \(\rightarrow\) Infected \(\rightarrow\) Symptomatic (often) \(\rightarrow\) Recovered and Immune (good!) or dead (bad!)
The idea behind vaccines
Infection
Healthy \(\rightarrow\) Infected \(\rightarrow\) Symptomatic (often) \(\rightarrow\) Recovered and Immune (good!) or dead (bad!)
Vaccination
Healthy \(\rightarrow\) Get Vaccine \(\rightarrow\) Symptomatic (sometimes) \(\rightarrow\) Immune (often)
What are vaccines
![]()
https://gladstone.org/news/how-do-vaccines-work - see also Fig 2 of Pollard et al.
How do vaccines work
- Induction of a protective, long-lived memory immune response
![]()
Pollard et al 2021, Figure 3
Challenges to make good vaccines
If you recover from measles, how long are you immune for?
If you get a measles vaccine, for how long are you protected?
If you recover from chlamydia, how long are you immune for?
If you get a chlamydia vaccine, for how long are you protected?
Evaluation of vaccines
How do we determine if vaccines are good?
- Safety: Reward/Benefit > Risk/Cost
- Efficacy/Effectiveness
- Economic viability/Cost-effectiveness
Vaccine Development
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Knipe et al Science 2020
Direct impact of vaccines
Vaccines (partially) protect those who receive them (direct/individual effect):
- Reduction in risk of infection/symptoms/hospitalization/death.
- Reduction in strength of symptoms.
Indirect impact of vaccines
Vaccines can also (partially) protect non-vaccinated contacts (indirect effect).
- Reduction of susceptibles in the population leads to overall reduced spread.
- Reduction of infectiousness potential leads to reduced spread.
![]()
Pollard et al 2021, Figure 5
Ways to evaluate vaccine impact
Measure it:
- Challenge studies
- Clinical trials
- Observational studies
Estimate it:
Measuring vaccine impact
Challenge studies
- One group receives the vaccine, the other placebo.
- Both groups are challenged with the pathogen under consideration.
- Well-controlled, can use small(ish) sample size.
- Somewhat unrealistic (e.g., high challenge doses).
- Direct effect only.
- Sometimes not feasible/ethical.
Clinical trials
- One group receives the vaccine, the other placebo.
- Groups are followed and outcome (infection/disease/etc.) recorded.
- Good balance between controlled and real-world setting.
- Usually needed for FDA approval.
- Only works if there are enough infections (not good for emerging pathogens).
- Can measure direct and indirect effects (but usually only direct).
- Expensive.
Observational studies
- Instead of assigning vaccines, we let individuals choose.
- Most “real”, least controlled.
- Can measure direct and indirect effects.
- Can be fairly inexpensive.
- Can lead to wrong conclusions.
Estimating vaccine impact
![]()
xkcd.com
Correlates of protection (CoP)
- Determining an immunological quantity that correlates with protection can make vaccine assessment easier.
- Finding correlates of protection (for vaccines) is very valuable (but can be tricky).
![]()
xkcd.com
CoP - SARS-CoV-2 Example
![]()
Khoury et al 2021 Nat Med
CoP - Influenza Example
![]()
Coudeville et al 2010 BMC MRM
Summary
- Vaccines are one of the best public health tools we have.
- Vaccines work by inducing protective immunity.
- Vaccines can provide direct and indirect protection.
- There are different ways to measure vaccine effectiveness.
Questions?
![]()
https://phdcomics.com/
- Slides: https://www.andreashandel.com/presentations/